Internal dehydration products of a hydrogenated sugar belong to the so-called “biomass-derived substances”, obtainable from natural products, being classified as “renewable resources”.
The expression “hydrogenated sugar” for the purposes of the present invention is understood to mean a sugar alcohol (also known as a polyol, polyhydric alcohol, polyalcohol, or glycitol) which is a hydrogenated form of carbohydrate, wherein a carbonyl group (aldehyde or ketone, reducing sugar) has been reduced to a primary or secondary hydroxyl group (hence the alcohol). Examples of hydrogenated sugars include in particular: (i) hexitols such as, for example, sorbitol, mannitol, iditol and galactitol, (ii) pentitols such as, for example, arabitol, ribitol and xylitol, (iii) tetritols such as, for example, erythritol and threitol.
The expression “internal dehydration product” is understood to mean any product resulting, in any manner, in one or more steps, from removal of one or more molecules of water from the original internal structure of a hydrogenated sugar such as those mentioned above. This may be advantageously internal dehydration products of hexitols, in particular of “dianhydrohexitols” or “isohexides” such as isosorbide (1,4-3,6-dianhydrosorbitol), isoidide (1,4-3,6-dianhydroiditol) or isomannide.
The expression “trivalent metal phosphates” for the purpose of the present invention means phosphates of trivalent metal (metal(III) phosphates) selected in the group consisting of boron phosphate, aluminum phosphate, iron phosphate, lanthanum phosphate and cerium phosphate.
The expression “solid catalyst” for the purpose of the present invention has to be understood as water insoluble materials which exist in various amorphous and/or crystalline states.
The expression “yield” for the purpose of the present invention has to be understood as:yield=(obtained product mass)/(theoretical product mass)=(moles number of obtained product)/(moles number of theoretical product)
The mass of obtained product is the mass synthesized. It is determined by weighing of the obtained product. The theoretical product mass is the mass of product corresponding to a yield of 100%. It must therefore be calculated from the mass of the reactants.
The conversion in % is a measure of the efficiency/activity of the catalyst. It is calculated by the amount of starting material added to the reactor (mol in) minus the amount of starting material found in the product mixture (mol out) divided by the amount of starting material (mol in) times 100.
The selectivity is a measure of the property of a catalyst to direct a reaction to a particular product (obtained product). The expression “selectivity” for the purpose of the present invention has to be understood as:selectivity=yield/conversion
In this formula, selectivity, yield and conversion are calculated on a molar basis. As an example, in a certain reaction, 90% of substance A is converted (consumed), but only 80% of it is converted to the desired substance B and 20% to undesired by-products, so conversion of A is 90%, selectivity for B 80% and yield of substance B 72% (=90%*80%).
Among the doubly dehydrogenated sugars, isosorbide is currently the one for which the largest number of industrial applications is being developed, or at the very least envisaged. Particularly, they relate to the formation of numerous pharmaceutical compounds, the preparation of food or cosmetic products, the production of plastics and polymers or the production of polyurethane, polycarbonate, polyesters and polyamides.
Acidic media are generally used for dehydrating sugar alcohol substrates to obtain internal dehydration products. Several processes for the production of internal dehydration products are known. All commercialized processes involve the use of concentrated homogeneous acids and organic solvents.
Current processes of internal dehydration of hydrogenated sugar are in liquid phase and carried out in standard batch reactors heated for example by oil in a double-jacket, fitted with a condenser and a receiver for the distillate. The standard conditions are gentle temperatures, e.g. between 100 to 200° C., and under vacuum, e.g. at 20 to 400 mbar. A liquid catalyst, like sulfuric acid H2SO4 at 1% by weight, is generally used.
This method results in high conversion levels, but requires the use of corrosive and non-reusable homogeneous Brönsted acid catalysts such as H2SO4. Thus, the need for one-use catalyst and corrosion resistant materials are major disadvantages of this method. Another big disadvantage is the high salt formation due to the neutralization of the homogeneous acids by bases and the costly disposal of those salts.
Alternatively, internal dehydration products can be produced by reacting an aqueous solution of hydrogenated sugar over heterogeneous catalysts. The solid nature of catalysts makes them easy to be removed and recovered from reaction mixtures and thus, to reduce the usage of solvents and environmentally adverse chemicals. Furthermore, there is no expensive working up procedure to get rid of the salt.
Goodwin et al. (Carbohydrates Res. 79:133-141, 1980) have disclosed a method involving the use of acidic-cation-exchange resin instead of concentrated, corrosive homogeneous acids, but with low yield of isosorbide synthesis.
U.S. Pat. No. 7,420,067 describes good yield of synthesis of internal dehydration products (about 50%) obtained in the presence of a solid acid catalyst at a temperature of about 150° C. to about 350° C. and under elevated pressure (from about 9 bar to about 140 bar). The solid catalysts used are inorganic ion exchange material selected from the group consisting of acidic ion exchange resins and acidic zeolite powders. However, these solid catalysts have the disadvantages of being not temperature stable in case of ion exchange resins and very expensive in the case of zeolites. In addition the zeolitic catalysts have the tendency to fast deactivation by blocking the pores and covering the acid sides by polymer and coke formation.
Other solid catalysts, such as metal (IV) phosphates, may present good performances for selective catalysis in the formation of mono- and bi-cyclic ethers from diols (Al-Qallaf Fawzia et al., J Mol Catal A:Chem 152:187, 2000).
As described in patent applications CN 101492457 and CN 101492458 metal (IV) oxides modified with H3PO4 and metal (IV) orthophosphates, as tin, zirconium and titanium phosphates, were already tested for selective dehydration of sorbitol to isosorbide in gas-phase. These tetravalent metal phosphates used as catalysts exhibit a good selectivity and a good yield in preparing isosorbide from sorbitol. However, the tetravalent phosphates present the inconvenience to be expensive, because their preparation requires a sequence of energy consuming subsequent processes, e.g. calcinations, refluxing for long times, hydrothermal treatment. More than that, some of them are toxic, such the tin phosphate. Other disadvantages are the high price of the metals such as Zr and Ti.
The objective of the present invention is to develop a simple, low cost, environmentally friendly and sustainable process for the production of internal dehydration products of hydrogenated sugar. This was achieved thanks to a special method implementing a trivalent metal phosphate (metal (III) phosphate) used as a catalyst.